The present invention relates to the general field of controlling clearance between the tips of rotor blades and a stationary bushing in a gas turbine.
By way of example, a gas turbine typically includes a plurality of stator blades disposed in alternation with a plurality of rotor blades in a passage for hot gases coming from a combustion chamber of the turbomachine. Over the entire circumference of the turbine, the rotor blades of the turbine are surrounded by a stationary bushing. Said stationary bushing defines a wall for the stream of hot gases passing through the turbine blades.
In order to increase the efficiency of the turbine, it is known to reduce the clearance that exists between the tips of the rotor blades of the turbine and the portions of the stationary bushing that face said blades to as little as possible.
To do this, means have been devised for varying the diameter of the stationary bushing. Generally, said means come in the form of annular pipes which surround the stationary bushing, and through which air is passed that is drawn from other portions of the turbomachine. The air is injected over the outer surface of the stationary bushing, causing the stationary bushing to expand or contract thermally, thereby changing its diameter. Depending on the operating speed of the turbine, the thermal expansions and contractions are controlled by a valve which serves to control both the flow rate and the temperature of the air fed to the pipes. Thus, the assembly consisting of the pipes together with the valve constitutes a tuning unit for tuning clearance at the blade tips.
Existing tuning units do not always make it possible to obtain highly uniform temperature over the entire circumference of the stationary bushing. A lack of temperature uniformity leads to distortions in the stationary bushing, which are particularly detrimental to the efficiency and the lifetime of the gas turbine.
Moreover, in existing tuning units, injection of air over the outer surface of the stationary bushing is generally not optimized, so that it is often necessary to draw a considerable amount of air in order to cool the stationary bushing. If too much air is drawn, this impairs the efficiency of the turbomachine.